Lymphocyte activation consists of multiple intracellular signaling processes. This complexity allows lymphocytes to regulate their function in various ways so that their differentiation and activation are properly carried out. Impairment in this complex system could lead to malfunctioning of the immune system, such as autoimmunity, allergy, and immunodeficiency. Nuclear factor of activated T-cells (NF-AT) is regarded as one of the most important transcription factors that controls lymphocyte activation. NF-AT is activated by Ca2+/calmodulin-dependent phosphatase calcineurin (CN). The interaction between NF-AT and CN is inhibited by FK506 and Cyclosporin A, which are broadly utilized for suppression of the immune responses. Recently, several cellular and viral proteins were also determined as the inhibitors of CN/NF-AT interaction. In the preliminary study, we identify a protein Reps1 that binds the lymphoid specific Src family kinase Lck and plays a critical role in NF-AT activation in lymphocytes. Reps1 is highly expressed in thymocytes and other lymphoid organs. Gene knockout of Reps1 in a chicken B-cell line DT-40 resulted in abolishment of NF-AT activation due to lack of NF-AT dephosphorylation. NF-AT activation was not restored even when cells were stimulated by pharmacological agents that bypass proximal signaling events. Further, transgenic mice expressing a mutant form of Reps1 in T cells showed significantly reduced IL-2 production by mature T cells. Together, the data indicates that Reps1 plays an essential role in lymphocyte activation. In this study, we will analyze the detailed mechanism of Reps1 function in vitro and its biological significance in vivo.